Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
FIG. 1 illustrates the operation of an optical touch screen sensor 10 described in U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260, and U.S. Patent Application Nos. 2002/0088930 A1 and 2004/0201579 A1 (the contents of which are incorporated into this specification by way of cross-reference). In this optical touch screen sensor design, integrated optical waveguides 11, 12 are used to launch an array of light beams 13 across a screen, then collect them at the other side of the screen and conduct them to a position-sensitive detector A touch event 14 (eg by a finger or stylus) is detected as a shadow 15, with position determined from the particular beam(s) blocked by the touching object. The touch screen sensors are usually two dimensional and rectangular, with two arrays (X, Y) of transmit waveguides along adjacent sides of the screen, and two corresponding arrays of receive waveguides along the other two sides of the screen. As part of the transmit side, in one embodiment a single optical source (such as a light emitting diode (LED) or a vertical cavity surface emitting laser (VCSEL)) launches light into a plurality of waveguides that form both the X and Y transmit arrays. In another embodiment, a separate optical source is used for each of the X and Y transmit arrays. In an existing design for the transmit side, the waveguide arrays guide light from the optical source to tows offenses 16 that expand the guided light beams in the horizontal (i.e. X, Y) plane, then collimate them in the horizontal plane as they are launched across the screen face. Collimation in the vertical plane may be achieved with an external vertical collimating lens (VCL), for example a cylindrical lens, not shown in FIG. 1. The receive side is essentially identical, and on each side the arrays of waveguides and lenses are positioned within the bezel of the screen. To minimise the width of the bezel, it is desirable for the transmit and receive elements to be as short as possible. For reasons of cost and ease of fabrication, it is highly preferred to form the waveguides and lenses out of a photo-patternable polymer material. Optical touch screens typically operate with infrared light to avoid interfering with the display, however visible light may be used if required.
The transmit and receive elements of the existing design as shown in U.S. Pat. Nos. 5,914,709, 6,181,842 and 2004/0201579 A1 encounter difficulties with collimation in the vertical plane, where for ease of assembly it is convenient to use a single VCL for all transmit or receive elements in each array along the sides of the optical touch screen. The placement of a VCL and a conventional transmit element 20 on a common base 28 is shown in FIG. 2a (plan view) and 2b (side view), with the end of substrate 26 butted against the back of VCL 23. Common base 28 could alternatively be placed on the top surface of transmit element 20. Transmit element 20 would normally be sandwiched between lower and upper cladding layers (as shown in FIG. 4b), but these have been omitted for simplicity. It can be seen that it is difficult for the entire curved end face 21 of transmit element 20 to be positioned at the focal plane 22 of VCL 23. Therefore while emerging rays 24 can be perfectly collimated in the vertical direction, this is not the case for rays 25. On the transmit side, the unavoidable spread of the beam in the vertical direction from incomplete collimation is simply a source of stray light. On the receive side however, the problem is potentially more serious because of the possibility of out-of-plane stray light entering the receive elements (this effect can be seen by reversing the direction of light rays 25 in FIG. 2b)
It can also be seen that optimal placement of focal plane 22 with curved end face 21 depends critically on gap 27 between the apex of curved end face 21 and the end of substrate 26. A simple approach in achieving the placement is to butt substrate 26 against VCL 23, which can be achieved in several ways well known in the art (for example a pick-and-place machine with a vision system). Nevertheless, distance 27 is governed by the amount that the substrate 26 protrudes past the apex of curved end face 21, and its accuracy depends on the tool used to cut the substrate. By means of alignment marks, a dicing saw typically can cut silicon wafers with an accuracy of approximately 10 μm, which may be sufficient for the present application. However for reasons of cost, it may be preferable to use plastic substrates, and unlike silicon where the only dimensional variable is thermal expansion (which is relatively easy to control), the dimensions of plastic substrates are also known to depend on humidity and thermal and mechanical history, which are far more difficult to control. For these reasons, accurate inter-layer registration is a known problem in the fabrication of multilayer plastic devices such as flexible displays.
Yet another problem with the transmit and receive elements of the existing design is that curved end face 21, being a reflective surface, must be an interface with a large refractive index difference, such as an air/polymer interface. Therefore when an upper cladding (highly desirable for optical isolation and mechanical protection of the waveguides) is being deposited, it has to be patterned so that it does not cover the curved end face, as discussed in U.S. Patent Application No. 2005/0089298 A1 (incorporated herein by reference in its entirety). However there is then a risk that the exposed curved end face could be damaged, for example during assembly of the optical touch screen sensor. The fact that curved end face 21 is an optical surface also means that gap 27 between transmit element 20 and VCL 23 cannot be filled with a transparent adhesive, which would aid in connecting the two components and prevent foreign matter from entering gap 27 and blocking the light.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.